Why is the thousand-year reign of Christ only the introduction to eternity?
The thousand-year reign is only the introduction because Revelation places a definitive thousand-year limit on the earthly kingdom, after which the kingdom economy moves into the eternal setting described in Revelation 21: a new heaven and a new earth. It is the beginning of eternity's promises, not the final state.
Les explains that even though the earthly kingdom is promised to Israel, Revelation puts a time limit on it of a thousand years, which is why it is called the millennium. That thousand-year reign of Christ is a definitive period, but it is merely the introduction for the eternal setting. When you get to Revelation chapter 21, there is a new heaven and a new earth, and the whole kingdom economy will just move right on into eternity. Les also stresses that eternity will not be boring or monotonous; it will be a viable exercise and a place of intense activity, though without all the problems, tribulations, and sorrows we have now.
And I saw a new heaven and a new earth: for the first heaven and the first earth were passed away; and there was no more sea.